Upper Greensand - Mid-Cretaceous Building Stone (Lead Author WGT)
The Upper Greensand Formation (Selborne Group) is mainly Late Albian to earliest Cenomanian in age, embracing the boundary between Lower and Upper Cretaceous (hence “Mid-Cretaceous” here). It is a mixture of fine to medium quartz sand, bioclastic sand and green glauconite grains deposited in a shallow sea during some 7 million years (c.105-98my). Fossil bivalves and tube worms are common, as are burrows (trace fossils). The Upper Greensand (UGS) has long been quarried for building stone in two main areas (a) NW Dorset: Shaftesbury to Okeford Fitzpaine, (b) SW Dorset - SE Devon.
South West Dorset - South East Devon (max 60m thick)
In this area, the UGS is divided into three Members, from base upwards: Foxmould, Whitecliff Chert, Bindon Sandstone. The Bindon Sandstone Member is a porous, light grey, medium-grained, calcareous sandstone of late Albian age. It consists of poorly-sorted grains of translucent quartz and chalky calcite sparsely cemented by powdery calcite. The proportions of quartz and calcite grains vary and varieties in which one greatly predominates over the other are common. Green glauconite and black iron oxides derived from it are common components of this stone. Bedding is evident in many blocks, usually poorly defined by variations in grain size, clast composition, degree of cementation and colour. See Whitchurch Canonicorum Church
Most building stone was extracted in the Branscombe area of SE Devon but used extensively in SE Devon (e.g. Exeter Cathedral) and West Dorset. In addition, the orange-brown siliceous Chert was used for walling, knapped and unknapped, much like the grey-black Flint from the Chalk.
The Eggardon Grit, a bioclastic limestone at the very top of the succession, is a thin local unit of the Bindon Sandstone occasionally used in walls near outcrops but also burnt for lime near Abbotsbury.
North East Dorset (max 60m thick)
In this area, the Upper Greensand is divided into four Members, from base upwards: Cann Sand, Shaftesbury Sandstone, Boyne Hollow Chert, Melbury Sandstone. The Shaftesbury Sandstone, hard and well-cemented by calcite, has been used for building around Shaftesbury and down the Stour valley as far as Blandford and Crawford Bridge, where the Tarrant joins the Stour. It was particularly useful for bridges, or the foundations of water mills, as well as the foundations of domestic buildings and churches. The UGS has been quarried recently on Manor Farm, Melbury Abbas, for new buildings in Shaftesbury. Melbury Sandstone, albeit younger, is virtually identical to Shaftesbury Sandstone.
UPPER GREENSAND (see image RIGHT)
Images A-C (XPL) show the main components: green pellets of glauconite & quartz sand grains variably coloured white to black (the PPL image D reveals more).
1. Glauconite - Many pellets of this green-yellow mineral, which gives the rock its name
2. Quartz - Depending on their sawn-through angle, grains of quartz can be white, grey or black under XPL, but the black can be confused with porosity cavities and iron-rich cement
Image D (PPL)
3. Quartz grains are seen more dimly and in the same faint colour
4. Cement - in this iron-rich rock the natural cement between the grains is revealed between the quartz and glauconite by the speckled black iron grains
5. Glauconite - Rusted (oxidised) iron in the glauconite is browner or grains brown-rimmed.
The Upper Greensand can be similar to Wardour/Chilmark Stone, for a comparison click here
Text by WGT & JT. Photos and images by JT and PB Feb 2017